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Commitf7e4d5c

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Doc: Clarify the role of aggressive VACUUMs.
Adjust the documentation's coverage of aggressive VACUUMs to make itclearer that aggressive vacuuming isn't always strictly necessary. It'spossible for non-aggressive VACUUMs to advance relfrozenxid/relminmxidwithout fail, given the right workload conditions.While this has always been true, it matters more with recent and pendingimprovements to VACUUM. These improvements make non-aggressivevacuuming more likely to advance relfrozenxid/relminmxid in practice.While this is an unrelated improvement to the docs, formally speaking,it still doesn't seem worth backpatching. So don't backpatch.
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‎doc/src/sgml/maintenance.sgml

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@@ -493,16 +493,17 @@
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will skip pages that don't have any dead row versions even if those pages
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might still have row versions with old XID values. Therefore, normal
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<command>VACUUM</command>s won't always freeze every old row version in the table.
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Periodically, <command>VACUUM</command> will perform an <firstterm>aggressive
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vacuum</firstterm>, skipping only those pages which contain neither dead rows nor
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any unfrozen XID or MXID values.
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When that happens, <command>VACUUM</command> will eventually need to perform an
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<firstterm>aggressive vacuum</firstterm>, which will freeze all eligible unfrozen
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XID and MXID values, including those from all-visible but not all-frozen pages.
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In practice most tables require periodic aggressive vacuuming.
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<xref linkend="guc-vacuum-freeze-table-age"/>
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controls when <command>VACUUM</command> does that: all-visible but not all-frozen
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pages are scanned if the number of transactions that have passed since the
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last such scan is greater than <varname>vacuum_freeze_table_age</varname> minus
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<varname>vacuum_freeze_min_age</varname>. Setting
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<varname>vacuum_freeze_table_age</varname> to 0 forces <command>VACUUM</command> to
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usethis moreaggressive strategy for all scans.
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alwaysuseitsaggressive strategy.
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</para>
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<para>
@@ -711,8 +712,9 @@ HINT: Stop the postmaster and vacuum that database in single-user mode.
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</para>
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<para>
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Aggressive <command>VACUUM</command> scans, regardless of
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what causes them, enable advancing the value for that table.
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Aggressive <command>VACUUM</command>s, regardless of what causes
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them, are <emphasis>guaranteed</emphasis> to be able to advance
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the table's <structfield>relminmxid</structfield>.
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Eventually, as all tables in all databases are scanned and their
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oldest multixact values are advanced, on-disk storage for older
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multixacts can be removed.

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